1. Field of the Invention
The invention generally relates to a fuel cell system, and more particularly to the improvement of a cooling system for a fuel cell stack. Also, the invention relates to a method for controlling circulation of a cooling liquid in a fuel cell system.
2. Description of the Related Art
A fuel cell generates electricity through a chemical reaction. More specifically, a fuel cell is structured such that an electrolyte is sandwiched between an anode and a cathode. Hydrogen or a hydrogen-rich reformate gas is supplied to the anode while oxygen is supplied to the cathode. An electrochemical reaction is then induced which results in the generation of electrical energy. A plurality of these fuel cells are usually combined to form a fuel cell stack. These fuel cells are classified into different types, such as phosphoric-acid fuel cells, solid oxide fuel cells, and polymer electrolyte fuel cells, according to the type of electrolyte they use, for example. Each type of electrolyte has an operating temperature that is best for performing its function. For example, the operating temperature of the electrolyte of a polymer electrolyte fuel cell (PEFC) is typically around 80° C. In order to run a fuel cell system while maintaining the optimum operating temperature of the electrolyte, a cooling system is provided for the fuel cell system. Polymer electrolyte fuel cells use a liquid including deionized water, unfreezable water and the like, for the cooling liquid.
The cooling system of a fuel cell includes a cooling liquid, a heat exchanger (such as a radiator) that reduces the temperature of the cooling liquid using outside air, a pump that pumps the cooling liquid, an internal passage that runs through separators provided in a fuel cell stack and through which the cooling liquid flows, a cooling liquid passage for circulating the cooling liquid between the heat exchanger and the fuel cell stack, and an ion filter for removing ions, and the like. This ion filter is made of an ion exchange resin, for example, and removes electrically conductive ions of metal, for example, that have leached into the cooling liquid from the heat exchanger, the fuel cell stack, the cooling liquid passage pipe wall, and the pump. Removing these ions prevents a reduction in insulating resistance between the fuel cell and the vehicle body earth.
JP(A) 2000-208157 discloses an example of a fuel cell system provided with such an ion filter. The invention disclosed in this publication proposes a structure in which there are a main coolant passage and a sub-coolant passage which is independent of the main coolant passage. The main coolant passage is used for cooling the fuel cell. The sub-coolant passage is used to circulate the coolant within a coolant tank through an ion filter, which is provided in the sub-coolant passage, in order to remove ions from the coolant.
When the fuel cell system is operating, it circulates the cooling liquid using a pump. Some of the cooling liquid is passed through the ion filter which removes ions, thereby lowering the ion concentration. When the fuel cell system is not operating, however, the pump is stopped so the cooling liquid is not circulated through the ion filter.
As a result, ions of metal, for example, leach into the cooling liquid from the heat exchanger, the fuel cell stack (ion exchange membrane and separator), the cooling liquid passage, and the like when the fuel cell system is not operating. As the ion metals, for example, leach into the cooling liquid, the concentration of ions in the cooling liquid increases. When the fuel cell system is not operated for an extended period of time, in which case the ion concentration in the cooling liquid increases substantially, it is necessary to remove the ions in the cooling liquid in order to reduce the ion concentration in the cooling liquid before restarting the fuel cell. Thus time is required before the fuel cell system can be started. A fuel cell system in a vehicle, in particular, is unable to be started up immediately because of the time that it takes to remove the ions. This means that time is required before the vehicle can be driven.